The present invention relates to the support of a piezoelectric vibrating reed and the constitution of a container in a piezoelectric vibrator.
A piezoelectric vibrator, such as a quartz resonator, is often supported in a box-type airtight container made of material, such as ceramic. Some piezoelectric vibrators sealed in box type containers are called surface mounted type (SMD type) vibrators. These vibrators are suitable for being mounted on printed circuit boards of electronic equipment and are often used as clock sources or the like in telecommunication equipment, portable electronic equipment, etc. A piezoelectric vibrating reed to be used for the vibrators is, for example, a strip rectangular plate type AT cut quartz vibrating reed, the outline of which is shown in a perspective view of FIG. 2. In FIG. 2, reference numeral 1 is a quartz vibrating reed, and reference numeral 11 is a quartz vibrating reed electrode film. A similar electrode film is provided on a back surface of the quartz vibrating reed 1. Reference numerals Ill and 121 are electrode lead lines of these electrode films. These lines are arranged in such a manner that they extend over both surfaces of an end of the quartz vibrating reed 1.
An example of a box type container for SMD type vibrators is composed of a lower case 2 shown in an exploded perspective view of FIG. 3 and a lid 5 shown in a perspective view of FIG. 4. The lower case 2 is a lamination of a first insulating substrate 21 and a second insulating substrate 22, both of which are made of ceramic. The first insulating substrate 21 is provided as a pattern in which bottom surface electrode terminals 211 for SMD type vibrators, and wiring patterns 213 connected with the bottom surface electrode terminals 211 by side surface patterns 212 and having through hole connecting sections 214 at important points, are metallized on a bottom surface.
On a top surface of the second insulating substrate 22, four terminals 221 for vibrating reeds are provided as metallized patterns. By means of a through hole (not shown in the drawings) provided within the top surface, the terminals 221 for vibrating reeds are connected with the wiring patterns 213 provided on the first insulating substrate 21. With regard to the four terminals 221 for vibrating reeds, two each are adjacent and the terminals which face each other in a longitudinal direction are connected. The quartz vibrating reed 1 is mounted on the lower case 2 and the electrode lead lines 111 and 121 of the quartz vibrating reed 1 are arranged on the terminals 221 for vibrating reeds. In such a position, the electrode lead lines 111 and 121 are adhered with a conductive adhesive, thereby simultaneously achieving continuity and support. At this time, it does not matter whether the quartz vibrating reed 1 is on the right side or wrong side, or what direction the longitudinal direction faces. The lid 5 is put on the lower case 2 where the quartz vibrating reed 1 is mounted, and the peripheral bottom surface is airtightly sealed utilizing the lower case 2 and glass having a low melting point. A hole 222 provided at the second insulating substrate 22 functions so that the lower case 2 does not touch the center of the quartz vibrating reed 1 which has a great vibration amplitude. Further, center grooves 223 are provided so as to avoid a short circuit of the adjacent conductive adhesives. Detailed constitution of the box type container described above is basically the same as that disclosed in Japanese Patent Application No. Hei 7-316064.
There are roughly two kinds of methods of supporting a piezoelectric vibrating reed. One (1) is a center supporting method where both of two short edges of a piezoelectric vibrating reed are glued to a lower case with a conductive adhesive or conductive and nonconductive adhesives. The other (2) is a cantilever supporting method where among the two short edges, only the edge where electrode lead lines are provided is glued to the lower case (the terminals 221 for vibrating reeds provided at the upper part of the lower case) with a conductive adhesive, and the other short edge is left not glued. For the purposes of supporting a piezoelectric vibrating reed so that the vibrating reed has sufficient strength to bear an impulse and avoiding a harmful influence of the stress (or distortion) caused by the support upon vibration of the piezoelectric vibrating reed, various proposals for improving both the supporting methods have been made heretofore.
The center supporting method (1) described above has the advantages that impact resistance sufficient to avoid damage of the piezoelectric vibrator can be obtained and a thin container can be used because the piezoelectric vibrating reed is stationary. However, it also has disadvantages as follows: due to a shrinkage caused by curing of an adhesive (heat treatment for hardening) or a difference in thermal expansion coefficient between the piezoelectric vibrating reed and a substrate of the container or an adhesive, a residual stress arises inside the piezoelectric vibrating reed of the completed piezoelectric vibrator, whereby it tends to cause such problems that an oscillation frequency differs from a target value, the temperature characteristic of the frequency and the temperature characteristic of a comfort index (CI) become turbulent, or spuriousness arises. In order to avoid these problems, in Japanese Utility Model Laid-Open Publication No. Hei 1-143516, it has been proposed that a nonconductive adhesive having a small shrinkage factor is applied to a bottom surface of each short edge of the vibrating reed so as to pass through the center support and a conductive adhesive (which generally has a large shrinkage factor) is applied to a top surface of each short edge of the vibrating reed, whereby the deterioration of characteristic which may be caused by the residual stress can be prevented. Further, in Japanese Utility Model Laid-Open Publication No. Hei 5-18121, it has been proposed that one of the short edges of the piezoelectric vibrating reed is supported by a conductive adhesive (having great hardness) and a soft silicon adhesive is applied to the other one of the short edges, whereby residual distortion is reduced or various stresses are absorbed.
In the cantilever supporting method (2) described above, the influence of stress resulting from the support upon the vibration of the piezoelectric vibrating reed may be considered negligible. However, since the cantilever piezoelectric vibrating reed is used, when a strong impulse is applied to the piezoelectric vibrator as it stands, there is a great risk of peeling of adhered parts, breaking the piezoelectric vibrating reed, or the like. In order to avoid such problems, there is a constitution such that a stopper to restrict displacement of the piezoelectric vibrating reed is provided near a short edge which is free from support. As a concrete example, Japanese Patent Laid-Open Publication No. Hei 8-330886 has disclosed a constitution in which a bolster section and a buffer section to serve as stoppers are arranged on the short edge so as not to be adhered. Further, Japanese Patent Laid-Open Publication No. Hei 8-186457 has disclosed the following art: a first adhesive is applied to a cantilever holding section of a quartz vibrating reed and temporary curing is applied to the adhered section in such a manner that the solvent splashes; the quartz vibrating reed is placed on the section and a second adhesive is applied thereto; by curing the section entirely in the manner described above, shrinkage caused by the adhesive curing is properly controlled; and by slightly lifting the other edge of the quartz vibrating reed which is not adhered up from a substrate, an adequate space is obtained between a surface of the substrate to be a stopper and the aforementioned other edge of the quartz vibrating reed.
Problems of the above-mentioned concrete examples of improvement which have been proposed heretofore will subsequently be described. With regard to a double application technology of adhesives disclosed in Japanese Utility Model Laid-Open Publication No. 1-14356 related to the center supporting method (1), since an operation of eliminating distortion is mainly applied to one of the short edges which is a limited part, there is a doubt as to whether or not it is effective. In the majority of SMD type vibrators which are placed on the market, although thickness or cost of the piezoelectric vibrator increases, distortion is eliminated by gluing each short edge of a quartz vibrating reed onto a flat spring. Further, it is expected that a combination of both soft and hard adhesives disclosed in Japanese Utility Model Laid Open Publication No. Hei 5-18121 will be effective. However, since the ""121 document does not enable a manufacturing method to be completed, there is a drawback to the realization. More specifically, a soft silicon adhesive is dissolved at approximately 350xc2x0 C., but heretofore a sealing temperature of the glass having a low melting point to seal the lid and lower case of a box type container was 370xc2x0 C. at the lowest. Thus, sealing work leads to dissolution of the silicon adhesive in the container, and ingredients generated splash and adhere to a surface of the quartz vibrating reed, thereby shifting a frequency. The attachment is not stable, so that there is a fear of omission. This causes variation of frequency with time. Further, gases generated may spoil an atmosphere or a degree of vacuum inside the container and may exert a harmful influence on the CI or the temperature characteristic. Moreover, there are no documents which have mentioned the difficulties described above. The art of providing the stoppers in the cantilever supporting method (2) which has been disclosed in Japanese Patent Laid-Open Publication No. Hei 8-330886 is also same as the above, as far as incompleteness of the manufacturing method is concerned. Also, the optimum material for the stoppers has not been mentioned.
An object of the present invention is to realize a piezoelectric vibrator housed in a box type container and having a center supporting or cantilever supporting piezoelectric vibrating reed which is capable of efficiently absorbing residual stress caused by adhesion and additive stress caused by temperature change and also having excellent quality and performance.
In the present invention, the center supporting constitution of the piezoelectric vibrating reed is such that one edge of the piezoelectric vibrating reed is fixed, supported, and connected by a conductive adhesive, and the other edge is glued to a lower case with a soft adhesive. Further, a lid and the lower case are sealed by sealing glass having a low melting point which is not dissolved by soft adhesive. Thus, shifting of frequency resulting from the sealing is slight and stress absorption characteristic of the soft adhesive is brought into full scope, whereby it is almost possible to completely alleviate the occurrence of residual stress which may be caused by mounting the piezoelectric vibrating reed. Further, there is hardly any additive stress which may be caused by a variation in temperature or the like, which means there is hardly any variation of frequency in assembly and there is no abnormality in the temperature characteristic. Due to a buffer effect of the soft adhesive, the piezoelectric vibrator is excellent in impact resistance and the piezoelectric vibrating reed is stationary inside the container, whereby a surplus space is not required and a thin case can also be realized. Incidentally, it is preferable to have the Young""s modulus ratio of over 500 of a hard conductive adhesive to a soft nonconductive adhesive.
Further, the cantilever supporting constitution of the piezoelectric vibrating reed is such that a stopper made of the soft adhesive is provided near the other edge of the piezoelectric vibrating reed and the case is sealed using the sealing glass having a low melting point which is not dissolved by the soft adhesive, as a result of which there is no shift of frequency which may be caused by the sealing and an excellent shock absorbing stopper in which the characteristic of the soft adhesive is brought into full scope is realized. Further, due to the cantilever supporting constitution, there is practically neither residual stress which may be caused by the support nor additive stress which may be caused by a variation of working temperature, and there is neither variation of frequency nor abnormality in temperature characteristic which may be caused by the assembly. Since a suitable clearance is provided between the piezoelectric vibrating reed and the stopper, the stopper is not responsive to normal acceleration which is encountered at the time of daily operation, and only when an impulse of over a specified strength which may cause damage of a supporting section is given due to the conductive adhesive, the piezoelectric vibrating reed and the stopper made of the soft adhesive colliding so as to prevent damage of the piezoelectric vibrating reed itself or the supporting section. Further, by controlling the quantity of the soft adhesive and the above-mentioned clearance, it is possible to control a total thickness of the piezoelectric vibrating reed so as not to be excessive and to be at an appropriate value.
Further, in both the center supporting and cantilever supporting constitution, it is possible to heighten the realization of the above by using a high molecular thermoviscous resin for the conductive adhesive.